Missing MathML elements and attributes

html5lib/sanitizer.py misses some MathML elements and attributes. In particular the semantics/annotation/annotation-xml elements required to fix a very old planet bug (http://www.maths-informatique-jeux.com/blog/frederic/?post/2013/01/22/Analysis-of-Lithium-s-algorithm#c142).

See https://github.com/html5lib/html5lib-python/pull/181
